
Re: Exercise of Programming with Mathematica
Posted:
Oct 20, 2013 11:18 PM


Instead of using ReplaceAll (/.), use Replace and specify the level.
In[1]:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {1}] Out[1]= {{b, a}, {d, c}}
In[2]:= Replace[{{a, b}, {c, d}}, {x_, y_} :> {y, x}, {0}] Out[2]= {{c, d}, {a, b}}
 Zhenyi Zhang <infozyzhang@gmail.com> wrote: > Here is a rule designed to switch the order of each pair of expressions in a list. > It works fine on the first example, but fails on the second. > In[1]:= {{a, b}, {c, d}, {e, f}}/.{x_, y_} :> {y, x} > Out[1]= {{b, a}, {d, c}, {f, e}} > In[2]:= {{a, b}, {c, d}}/.{x_, y_} :> {y, x} > Out[2]= {{c, d}, {a, b}} > Explain what has gone wrong and rewrite this rule to correct the situation, > that is, so that the second example returns {{b, a}, {d, c}} > > My solution is the most stupid one. > > {{a, b}, {c, d}}/.{{x_, y_}, {w_, t_}} :> {{y, x}, {t, w}} > > May I ask any elegant solutions? > > Thanks.

